The present invention relates to registration of plural image exposures formed on a photoreceptor belt in a single pass by a plurality of imagers and, more particularly, to improvements in registration by sensing registration marks associated with each image exposure.
Printers are known in the art wherein an imager, such as a Raster Output Scanner (ROS) or a linear LED print bar, is positioned in an optical scan system so as to write an image on the surface of a moving photoreceptor belt. In a ROS system, a modulated laser beam is swept across the photoreceptor surface after being directed against the facets of a rotating polygon. Each sweep exposes a raster line to a linear segment of a video signal image. The LED print bar generally consists of a plurality of light emitting diodes or pixels arranged in a linear array on a common substrate. Each LED in the linear array is used to expose a corresponding area on a moving photoreceptor to an exposure value, defined by the video data information applied to the drive circuits of the print bars. With use of either imager, the photoreceptor is advanced in the process direction to provide a desired image frame by the formation of sequential scan lines.
In a color xerographic printer, a plurality of imagers, either all LED print bars or all ROSs or a combination thereof, are positioned adjacent a photoreceptor belt surface and selectively energized to create successive image exposure until a series of image frames, each of a separate color image, is formed. If two imagers are used, the system typically provides one highlight color and one black color. For full color, four imagers are used, one for each of the three basic colors and a fourth print bar for black images.
Each color image must be registered in both the photoreceptor process direction (pitch and skew registration) and in the direction perpendicular to the process direction (referred to as the fast scan or transverse direction).
There are several prior art techniques for registering LED print bars in the lateral direction. Co-pending application U.S. Ser. No. 07/807,931, assigned to the same assignee as the present invention, discloses LED print bar registration techniques which utilize belt holes and associated sensors to generate registration correction signals which are used to drive stepper motors which provide transverse and rotational motion to an associated image bar. Another co-pending application U.S. Ser. No. 07/635,835, assigned to the same assignee as the present invention, discloses lateral registration techniques which include physical shifting of the print bar by operation of piezoelectric stacks. A still further co-pending application, U.S. Ser. No. 07/862,150, assigned to the same assignee as the present invention, discloses lateral registration of print bars by use of pairs of oppositely aligned holes formed on the photoreceptor, beneath which are placed sensors which detect light from selected print bar pixels. Another co-pending application, U.S. Ser. No. 07/807,927, assigned to the same assignee as the present invention, discloses process direction (skew) registration techniques related to those of the '150 application but directed to ROS imagers. The contents of these four applications are hereby incorporated by reference.
For some systems, it may be desirable to use a combination of imagers; e.g. a ROS imager is used to form at least one of the image frames and LED print bars are used to form the remaining frames. One reason for implementing this hybrid imaging system is the more compact nature of the imaging space enabled by using print bars which occupy less space than the counterpart ROS unit. The compact nature of the LED imager enables upgrade of existing products with the addition of highlight color. The above listed disclosures, while providing registration for an all ROS or an all print bar imaging system, do not provide for registration of a hybrid system.
The present invention is, therefore, directed towards a method and apparatus for registering a plurality of images in a process direction in a hybrid ROS/LED print bar imaging system. The method and apparatus, however, may also find utility in registering an all ROS imaging system. The method and apparatus, however, may also find utility in registering an all ROS imaging system. More particularly, the present invention is directed towards an apparatus for registration of multiple image exposure frames on a photoreceptor belt, moving in a process direction, during a single rotation of the photoreceptor including:
a photoreceptor belt adapted to accommodate the formation of an integral number of image exposure frames, said belt having first and second registration targets on opposite sides of the belt width in non-image areas, each target comprising a transparent V-shaped target with the apex of each V aligned along a common line parallel to the process direction,
at least one Raster Output Scanner (ROS) unit associated with the formation of one of said image exposure frames, said ROS unit forming a plurality of projected scan lines in a fast scan (transverse) direction across the belt width, said scan lines beginning and ending at points outside of the image exposure frame and crossing over said registration targets, when said targets move therepast,
detecting means associated with each of said targets, said detecting means detecting the light passing through said targets from said ROS as said belt carries said targets past said ROS,
control means for comparing output signals generated by said detecting means to establish whether coincidence exists between signals generated by each pair of associated detectors, and
means for skewing the projected scan lines formed by said ROS until said detected signals are coincident.